Most vapor-intrusion constituents are Volatile Organic Compounds (VOCs), which are carbon based. Hydrocarbons are composed, not surprisingly, primarily of hydrogen and carbon, and their names are tied to the number of carbon atoms they contain. The simplest group, alkanes, are hydrocarbons with carbon atoms that are singly-bonded to one another, as follows:
The first four are shown below (note the number of carbon “C” atoms) :
The root syllable “eth” in tetrachloroethene, trichloroethene, and dichloroethene indicates that they have two carbon atoms, but the “ene” suffix tells us that the carbons are doubly bonded, which allows each carbon atom to attach to only three other atoms, instead of four. And of course, the prefixes “dichloro”, “trichloro”, and “tetrachloro”, indicate that the molecules have two, three, and four chlorine atoms, respectively, as shown below:
Chlorine doesn’t attach to hydrocarbons in nature, but it drops off of chlorinated compounds during chemical breakdown. Consequently, PCE, TCE, DCE, and VC all are manufactured, but PCE breaks down to TCE, DCE, and VC.
DCE is especially interesting for vapor intrusion, because there are three different ways to configure the chlorine atoms. 1,1-DCE, as shown below, is commonly manufactured, but cis-1,2-DCE and trans 1,2-DCE usually result from the breakdown of TCE in the subsurface. Therefore, cis-1,2-DCE and trans-1,2-DCE in indoor air generally indicate the presence of vapor intrusion, and the ratio of indoor-to-subsurface cis-1,2-DCE serves as a benchmark for other vapors. For example, if cis-1,2-DCE is present in indoor air at a concentration of 1 microgram per cubic meter (ug/m3) and in subslab soil gas at a concentration of 1,000 ug/m3, other compounds from vapor intrusion should have more or less the same ratio. Suppose benzene is present in the same subslab soil gas at a concentration of 5,000 ug/m3, one would expect vapor intrusion to contribute approximately 5 ug/m3 to indoor air. But if benzene is present in indoor air at a concentration of, say, 30 ug/m3, most of it is probably from background sources, i.e., indoor substances or from outdoor air. In our experience, cis-1,1-DCE is far more abundant than trans-1,2-DCE, and is therefore a better vapor-intrusion tracer.
As to their uses, PCE and TCE are widely used for removing grease and oil. Much of the PCE Cox-Colvin encounters was used to clean metal parts during manufacturing, and it’s the main ingredient of most parts-cleaners available in the automotive department. PCE is also the most common dry-cleaning fluid, so there are countless potential sources of contamination. TCE is also used for industrial degreasing, but in somewhat smaller volumes than PCE. Both are common ingredients in adhesives, paints, and a number of other substances, as discussed in the New Jersey DEP 2016 summary of background contamination, and Appendix I of New Jersey’s 2018 Vapor-Intrusion Guidance.
As mentioned earlier, cis-1,2-DCE and trans-1,2-DCE are not commonly found in products, which is probably why it is not listed among the TO-15 air compounds, or for that matter, the Appendix IX groundwater compounds. Most labs routinely report cis- and trans-1,2-DCE with the other TO-15 analytes, but verify that in advance, especially if you’re working with chlorinated solvents. None of the three configurations of DCE (isomers) are widely used as solvents, but they are used to synthesize plastics, and their potential to outgas from plastics might explain their occasional presence as background in indoor air.
VC, according to the ATSDR TOXFAQs website, “has been found in at least 616 of the 1,662 National Priority List sites…” Like DCE, VC is most widely used in the manufacture of plastics, but it has also been used as an aerosol propellant, a refrigerant, and even an anesthetic!!
According to US EPA’s 2011 VOCs in Background report, PCE and its breakdown products have been detected in background (i.e., presence is due to everyday materials in the household) in North American residences in the following frequencies: PCE 62.5%, TCE 42.6%, cis-1,2-DCE 4.9%, 1,1-DCE 13%, and VC 9.2%. Obviously, PCE and TCE are so common in background that their presence in indoor air does not necessarily indicate vapor intrusion, and like chloroform, benzene, and a number of other compounds, their presence in indoor air must be interpreted in light of multiple factors. But again, the ratio of indoor-to-subslab concentrations should resemble those of cis-1,2-DCE, if it’s present in soil gas.
EPA’s Vapor Intrusion Screening Levels (VISLs) for the PCE family of compounds, assuming an Excess Lifetime Cancer Risk of 1 in 100,000 (ELCR 10-5), and a Hazard Index of 1, in a residential setting is as follows:
TCE does not have the lowest screening level of the lot, but the fact that it is associated with fetal heart defects after short exposures makes it a particular concern, as discussed in earlier newsletters.
PCE and its breakdown products are ubiquitous to vapor intrusion. An understanding of their behavior, and especially the role of cis-1,2-DCE, is key to unraveling the vapor-intrusion puzzle at commercial/industrial sites.
Mort Schmidt is a Senior Scientist with Cox-Colvin & Associates, Inc. He received his BS and MS degrees in Geology and Mineralogy from The Ohio State University, and has been a Cox-Colvin & Associates employee since 1997. His areas of expertise include vapor intrusion and contaminant investigation and analysis, and he currently serves as Cox-Colvin's Practice Leader - Vapor Intrusion Services. Mort is a Certified Professional Geologist with AIPG and is a registered Geologist in Indiana. Craig Cox is a principal and co-founder of Cox-Colvin & Associates, Inc., and holds degrees in geology and mineralogy from the Ohio State University and hydrogeology from the Colorado School of Mines. Mr. Cox has over 30 years of experience managing large environmental project implemented under CERCLA and state voluntary action programs. In addition, Mr. Cox has developed a variety of software products including Data Inspector, an internet-enabled environmental database application. Mr. Cox is a Certified Professional Geologist (CPG) with AIPG and is a Certified Professional (CP) under Ohio EPA's Voluntary Action Program.